1. Field of the Invention
The present invention relates to an apparatus and method for controlling a polarization state of a radiation beam.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus may be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which may be referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern may be transferred on (part of) the substrate (e.g., a glass plate), e.g., via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate.
Instead of a circuit pattern, the patterning means may be used to generate other patterns, such as a color filter pattern or a matrix of dots. Instead of a mask, the patterning device may comprise a patterning array that comprises an array of individually controllable elements. An advantage of such a system compared to a mask-based system is that the pattern can be changed more quickly and for less cost.
A radiation beam that applies a pattern to a substrate (e.g., a resist coated substrate) may be a continuous beam, or may be a beam comprising a plurality of pulses. For instance, in one example the radiation beam may comprise radiation beam pulses. The radiation beam pulses may be provided directly by the radiation beam source, or by selective allowing or prevention of the passage of a continuous radiation beam.
When using a pulsed radiation beam to apply a pattern to a substrate, a plurality of pulses may need to be incident on the same area of the substrate in order to apply the pattern, or a part of the pattern, to the area of the substrate. The energy of each pulse may vary by ±10% of a mean value for each pulse. However, the variation in pulse energy can be taken into account using a fast control algorithm and control electronics. The variation in pulse energy is also often averaged out by the fact that a plurality of pulses is used to pattern a given area of a substrate. For instance, to achieve a required radiation dose on a particular area of the substrate, between forty and sixty radiation beam pulses may be required. The resultant fluctuation in the cumulative dose may only vary by +0.1% a mean value. Thus, in some applications, the variation in energy of pulses of a pulsed radiation beam may not have much of an effect on the application of a pattern or patterns to a substrate.
In some applications, including maskless lithography apparatus and methods that use a mirror array or the like, it may be desirable to use only a single pulse of a pulsed radiation beam to provide an area of the substrate with a required dose of radiation. Since only a single pulse will be used to provide the required dose, the above mentioned variation in energy for each dose may result in a similar (e.g., ±10% of the mean) variation in the radiation dose applied to the area of the substrate. Such a large variation in the dose of radiation may result in an unacceptable variation in the line width of patterns applied to the substrate. In order to achieve acceptable control of the line width of patterns applied to the substrate, variation in the energy of the radiation beam pulses is preferably less than 10% of a mean value, for example, at most 0.5% of a mean value. However, current radiation beam sources are not capable of supplying a pulsed radiation beam with such a low variation in pulse energy.
A possible solution to the variation in the energy of pulses of a radiation beam is to trim (or control) the energy of an individual pulse using a fast detector and a fast optical shutter, for example detectors and shutters having nano-second response times. For instance, a suitable shutter may be a Pockels cell. As will be known in the art, Pockels cells are a common electro-optic device used for light modulation. A Pockels cell may be used to change the polarization state of a radiation beam which passes through the Pockels cell. By combining the Pockels cell with an optical analyzer, an optical switch may be created. For example, by rotating the polarization state of a radiation beam by 90° degrees, the radiation beam may be selectively allowed or prevented from passing through the optical analyzer and onto or through other elements of the lithographic apparatus.
One problem associated with the use of a Pockels cell, or other electro-optic devices, is its susceptibility to changes in the environment. For instance, a small change in temperature of the Pockels cell may have a significant effect on the control or changing of the polarization state of the radiation beam which passes through the cell. If the polarization state of the radiation beam were not controlled accurately enough, too much or not enough radiation may pass through the optical analyzer and on to or through other elements of the lithographic apparatus. For example, in one situation, it may be desired to pass as much light as possible through the optical analyzer and on to or through other elements of lithographic apparatus. In this situation, a change in the polarization state of the radiation beam caused by a change in temperature of the Pockels cell may reduce the intensity of the radiation beam which passes on to or through other elements of lithographic apparatus. In another example, it may desired to prevent any part of the radiation beam from passing on to or through other elements of the lithographic apparatus In this situation, a change in the polarization state of the radiation beam caused by change in temperature of the Pockels cell may result in a portion of the radiation beam passing through the optical analyzer and on to or through other elements of the lithographic apparatus. It is desirable to reduce or eliminate the drift (or deviation) in the polarization state of the radiation beam due to, for example, changes in the temperature of the device which controls the polarization state.